1. Field of the Invention
The present invention relates to an hydraulic control circuit for a continuously variable transmission and relates particularly, but not exclusively, to such a circuit for what is commonly referred to as a xe2x80x9cfull toroidalxe2x80x9d continuously variable transmission. (CVT).
2. Discussion of Prior Art
Such CVT""s comprise an input shaft for receiving power from, for example, an internal combustion engine and has mounted thereon a pair of spaced apart input discs and a pair of output discs mounted back to back between said input discs. The input discs rotate with the shaft but the output discs are mounted for free rotation on the shaft by means of a bearing or some such similar device. The confronting faces of the input and output discs are contoured to provide a concave surface or face, which mirrors that of the face facing it. The faces are either formed in a full or half toroidal manner and provide the surfaces between which a plurality of rollers are positioned for transmitting power between the input and the output discs. In at least the full toroidal design, the discs are hydraulically end loaded to ensure that traction is maintained between the discs and the roller. Additionally, the rollers themselves, whilst having a certain degree of freedom of movement are subjected to some degree of positional influence by an hydraulic actuator employing both the higher and the lower pressures within an hydraulic control circuit, such as disclosed in PCT GB/00956 or British Patent No. 2282196
The above arrangement is illustrated in FIG. 1, in which items 12, 14 are the input discs, item 16 is the input shaft and items 13, 20 are the output discs. The rollers are shown at 22 and a double acting hydraulic piston 24 employs the higher and lower pressures within an associated hydraulic circuit to influence the position thereof, thereby to vary the transmission ratio through the transmission. Hydraulic end load is provided by means of an hydraulic chamber 26 which, when supplied with hydraulic fluid under pressure acts to load input disc 14 towards the other input disc 12, thereby ensuring traction is maintained.
An hydraulic control suitable for the above arrangement is described in our own PCT application number PCT GB/00956, the main circuit of which is shown in FIG. 2 attached hereto. Whilst this arrangement need not be discussed in detail herein, it will be appreciated that valves 99 and 100 are connected in parallel and are operable either individually or together to vary the pressure within the hydraulic control circuit in order to influence the position of the rollers 22 and the hydraulic pressure being applied to either of two clutches 37, 43. The control is independent i.e. varying the hydraulic pressure in the clutches has no affect on the position of the rollers and vice versa.
It is an object of the present invention to provide an hydraulic control circuit for a continually variable transmission, which improves on the above design, by providing a control circuit capable of more rapid response to adverse operating conditions.
Accordingly, the present invention provides an hydraulic control circuit for a CVT transmission comprising:
first and second hydraulic supply pipes;
first and second hydraulic pumps, PL, PR associated with said first and second supply pipes respectively for pumping hydraulic fluid therethrough and for raising its pressure;
a first hydraulic pressure control valve V1 for controlling the pressure of hydraulic fluid to be supplied to the roller control of the variator;
a second hydraulic pressure control valve V2 for controlling the pressure of hydraulic fluid to be supplied to a clutching arrangement of the transmission (backpressure control);
characterised in that said valves V1, V2 are connected in flow series and by a first fluid directing valve means (S1, S2) for directing flow from each pump PL, PR to a first point upstream of valve V1 or to a second point downstream of valve V1 but upstream of valve V2.
Advantageously, the first fluid directing valve means comprises two two-way valves S1, S2 and each having first and second outlets, each first outlet being connected for supplying fluid to said first point (upstream of V1 ) and each second outlet being connected for supplying fluid to said second point (between V1 and V2).
Preferably, valve V1 comprises a pressure raising valve for controlling pressure upstream thereof for supply to said variator and in which any flow through said valve V1 is combined with any flow being directed directly to said second point for subsequent supply to said second (clutch control valve) V2.
Advantageously, the circuit further includes flow restriction means R for restricting the flow of fluid in the direct supply between the fluid directing valve (S1, S2) and said second point such that the resistance in each branch is substantially equal to the total resistance in the supply route through valve V1 .
In a particularly advantageous arrangement, the circuit includes a further restrictor r within the supply to valve V1 and the total resistance of r and the resistance in Valve V1 is substantially equal to the resistance R and any resistance within each branch between the fluid directing valve means (S1, S2) and the second point (P2).
Conveniently, the circuit further includes a vehicle deceleration monitor and switching means operable to switch the first fluid directing valve means to cause all the fluid from pumps PL, PR to be directed to valve V1 upon detection of vehicle deceleration.
Advantageously, the circuit includes switching means operable to switch the first fluid-directing valve means to cause all the fluid from pumps (PL, PR) to be directed to said second point rather than said first point.
In one arrangement the secondary fill point SFP is upstream of said first fluid directing valve means (S1, S2).
Advantageously, the circuit includes a flow restrictor rL, rR in the supply to each clutch, thereby to maintain a predetermined pressure within the supply leading thereto. Preferably, each clutch supply includes a clutch fill valve FL, FR between an associated pump and said clutch, said valve receiving fluid flow from said associated pump either via a primary fill point PFP downstream of said second point but upstream of valve V2 or from a secondary fill point SFP downstream of said pumps but upstream of said first point P1.
In one arrangement the secondary fill point SFP is upstream of said first fluid directing valve (V3).
Whenever the circuit is provided with clutches it preferably includes a dump valve (EL, ER) for each clutch acting in a first position to direct flow to said associated clutch and in a second position acting to allow fluid to drain therefrom but preventing fluid flowing thereto.
Advantageously, the circuit includes control means for controlling valves FL, FR so as to cause fluid to be supplied from the primary fill point PFP during a clutch fill step and from the secondary fill point SFP during a clutch engage step.
Additionally, the circuit may include control means for controlling the first fluid directing valve (S1, S2) to direct fluid from both pumps P1, P2 to a particular clutch CL, CR via said secondary fill point SFP .
Advantageously, the circuit further includes a fluid accumulator for receiving fluid flow once clutch engagement has been completed.
Preferably said accumulator receives fluid from a tertiary filling point TFP upstream of valve V2 but downstream of the primary filling point PFP.
Advantageously, when the above accumulator is employed, valve V2 comprise a solenoid valve.
In a particularly advantageous arrangement the circuit further including a variable rate relief valve and in which valve V2 comprises a solenoid valve, said valve V2 in a first position acting to direct flow through said valve and to a sump and in a second position acting to direct fluid to said variable rate relief value.
Preferably, said variable rate relief valve comprises a spring loaded pressure relief valve having a xe2x80x9ctiming restrictorxe2x80x9d circuit for receiving a portion f the flow and directing it to the spring side of the accumulator, thereby to assist the spring effect and increase the pressure within the hydraulic circuit.
Advantageously, said pressure relief valve includes a drain for draining any fluid accumulated on the spring side of said valve.
In a convenient arrangement, the circuit includes an hydraulic end load mechanism of a continuously-variable-transmission and said circuit includes means (highest wins valve) for supplying said end load mechanism with the higher of the two pressures created by pumps (PL, PR).
Preferably, the above arrangement includes a pressure sensitive valve (HW) connected for receiving hydraulic fluid from both pumps PL, PR and for directing only the fluid at the higher pressure to the end loading mechanism.
In addition to the above end load arrangement, the circuit may be provided with means for supplying said end load mechanism with fluid at the lowest pressure within the circuit as an alternative to fluid at the higher of the two pressures created by pumps PL, PR.
Advantageously, the hydraulic control circuit further includes switching means for switching the supply fluid being supplied to the end load mechanism between that at the lower pressure and that at the higher pressure.
In an alternative mode, the present invention comprises a continuously variable transmission having an hydraulic control circuit as described above.